Python sol

solution/3700-3799/3700.Number of ZigZag Arrays II/README.md

@@ -97,6 +97,39 @@ source: 第 469 场周赛 Q4
 
 ```python
 
+class BinaryIndexedTree:
+    __slots__ = "n", "c"
+
+    def __init__(self, n: int):
+        self.n = n
+        self.c = [0] * (n + 1)
+
+    def update(self, x: int, delta: int) -> None:
+        while x <= self.n:
+            self.c[x] += delta
+            x += x & -x
+
+    def query(self, x: int) -> int:
+        s = 0
+        while x:
+            s += self.c[x]
+            x -= x & -x
+        return s
+
+
+class Solution:
+    def countMajoritySubarrays(self, nums: List[int], target: int) -> int:
+        n = len(nums)
+        tree = BinaryIndexedTree(n * 2 + 1)
+        s = n + 1
+        tree.update(s, 1)
+        ans = 0
+        for x in nums:
+            s += 1 if x == target else -1
+            ans += tree.query(s - 1)
+            tree.update(s, 1)
+        return ans
+
 ```
 
 #### Java